These studies are concerned with the role of beta-alanine in the maintenance of melanistic polymorphism in Drosophila. Emphasis is placed on the effects of beta-alanine on aggressive territorial behavior and sexual isolation with reference to differential selection of microhabitats by variously pigmented genotypes. Flies newly extracted from free-ranging populations, and melanistic mutants maintained in the laboratory are used. Especially examined is the role of chemoreception in arena behavior. Competitive population jars and a large observation chamber are used. Experiments are performed to determine the mechanism by which the chemistry involved in pigmentation is related to behavioral modification. Since beta-alanine plays a key role here, particular attention is paid to this amino acid. Attention is given to the influences of beta-alanine on ultrastructures, growth, and glucose catabolism. Electron microscopic auto-radiography is employed.